A laser look at turbine engine refurbishing

  • 01-May-2009 11:31 EDT
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Quang Tan (right), President, and Son Dao, Operations Manager, TL Precision Welding, stand adjacent to a refurbished gas turbine component laser-processed with the company's Laserdyne 790 BeamDirector system.

Turbine repair and refurbishing is an industry niche with promising growth, mainly because of the sheer number of both land-based and aerospace turbine engines in operation and the constant need for engine maintenance. One aerospace turbine engine manufacturer alone reports 25,000 engines in active service, all of which are continuously monitored for periodic maintenance.

High-value engine components—such as turbo combustors, nozzle guide vanes, blades, and other transition parts—for both land and aerospace engines, are regularly refurbished and put back into service. While becoming expert at refurbishing such components, Houston-based TL Precision Welding, a 12-person contract shop, also positioned itself with new equipment and skills to handle laser machining work, using Laserdyne systems for drilling difficult-to-machine materials such as Inconel 617, titanium, and Hastelloy X.

A significant amount of engine overhaul calls for the repair and refurbishing of engine components’ airflow holes. Large combustor components have thousands of these small airflow holes of various sizes and shapes. The holes are designed to maximize engine thrust by selectively cooling critical components, and are precision-drilled using laser processes in carefully plotted, complex patterns over the part’s contoured surfaces. The holes also are contoured and angled to the part surface to maximize airflow, reduce engine noise, and minimize fuel requirements. Hole angles vary from 90° to just 20° to the surface and require a high degree of complex and agile positioning and dimensional precision.

The challenge with refurbishing airflow holes in such complex parts is that they must be re-created in the same position and with the identical precision size characteristics as the original part when new, according to Quang Tran, President of TL Precision.

“Re-creating precision holes in a used part is often more difficult than generating precision holes in a new part,” said Tran. “In re-creating the hole, there’s no margin for error because most of these holes are positioned closely together and have diameters as small as 0.02 in and require tolerance accuracies as tight as ±0.002 in. To position and laser process each of these holes accurately—without damaging the part—requires the very best multi-axis laser machining technology and a high level of operator skill.

“Our Laserdyne 790 BeamDirector systems are often the same models used by OEMs to make the parts originally, so there are both hardware and software compatibility, which helps facilitate the refurbishing process," said Tran. "Using the same programming coordinates as when the part was processed new, our operators re-establish the hole location and align the laser beam to it. Our operators verify hole location and beam alignment so there is no chance of error using the ‘through-the-lens’ viewing feature, which magnifies clearly all hole features [magnification is 45 times]. Every hole is thereby processed and verified individually.”

TL uses percussion drilling and trepanning processes to re-create cooling holes. Most frequently used is percussion drilling, which is a method that generates a hole with only the laser’s focused beam diameter. A single pulse or a series of pulses remove the material in the beam path, with an assist of co-axial flowing gas until the hole is re-created. For existing holes, slag, exhaust refuse, and other foreign material that have built up in the holes during engine operation are removed cleanly and accurately.

Trepan drilling is a similar but less frequently used method in engine refurbishing processes. Trepanning does not rely on the laser beam size to give the final dimensions of the finished hole. With the part held stationary, the laser beam is moved with simultaneous multiple action motion to create the hole by cutting the periphery of the shape.

Tran says that while this hole-refurbishing process often requires a manual point-and-shoot technique, it is consistently accurate and faster than any other known method. “That, plus with our Laserdyne 790 system we have current technology in our laser systems and trained operators that can easily process new parts when we get this kind of order. We are using all of Laserdyne’s automatic and quality features that are designed into its systems.”

TL’s laser system operators have particularly taken to Laserdyne’s Automatic Focus Control (AFC) feature. “What we often had to do manually to re-create a hole, we can do automatically to make new holes in new parts with AFC,” said Tran.

AFC guides the motion system, maintaining critical focus position and following the contour of the part regardless of slight surface irregularities. With AFC, all machine axes react to sensing of the part surface, creating unlimited R-axis correction with high speed and unmatched sensitivity. The R-axis is a virtual axis and refers to the direction that the laser beam is pointing toward at any given time during processing.

Another important capability with TL’s laser systems is a variation of percussion drilling called “drill-on-the-fly,” a feature in which laser beam pulses are delivered to the part by the stationary laser while the part is rotated. The hole placement is a function of rotational speed and laser pulse frequency.

If multiple pulses are required, drill-on-the-fly software developed by Laserdyne engineers is used to synchronize the movement of the part to the laser pulses, ensuring that multiple pulses are delivered to the exact location required. By changing the laser pulse energy, pulse count, or lens focal length, the characteristics of the drilled hole size and taper can be controlled to meet the requirements of the part. Drill-on-the-fly software also allows changes of the pulse shape during the process to improve hole geometry.

“We continue to build on our laser-processing knowledge with these systems,” said Tran.

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